An image sensor comprises an array of pixels formed from a semiconductor wafer. Charges are generated in each pixel according to the light received during an acquisition period, and the number of generated charges is read during a read period. In certain image sensors, the pixels are associated with memory regions where the generated charges are periodically transferred to be read later on.
A problem is that light may reach the memory areas between the transfer and read time, and generate electron/hole pairs therein. This modifies the number of stored charges, which decreases the image quality. This problem is in particular raised for back-side illuminated image sensors. Structures of optical isolation of the memory regions have been provided, such as that described in United States Patent Application Publication No. 2016/0118438, which provides, in relation with its FIG. 2, for surrounding each memory region with opaque tungsten walls and providing on the back side a tungsten shield layer. However, the opaque walls and the shield layer are not contiguous and light may pass therebetween and reach the memory region. Such structures thus let through part of the light. These structures further have various manufacturing and implementation problems.
It is thus desired to have a back-side illuminated image sensor comprising memory regions efficiently protected from light, as well as a method of manufacturing such a sensor.